lunar skylight polarization signal polluted by urban lighting
DESCRIPTION
Lunar Skylight Polarization Signal Polluted by Urban Lighting. Christopher Kyba 1,2 , Thomas Ruhtz 1 , Jürgen Fischer 1 , Franz Hölker 2 1 Freie Universität Berlin 2 Leibniz-Institute of Freshwater Ecology and Inland Fisheries 11th Dark Sky Symposium, Osnabruck Oct 6, 2011. - PowerPoint PPT PresentationTRANSCRIPT
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Lunar Skylight Polarization Signal Polluted by Urban Lighting
Christopher Kyba1,2, Thomas Ruhtz1, Jürgen Fischer1, Franz Hölker2
1Freie Universität Berlin2Leibniz-Institute of Freshwater Ecology and
Inland Fisheries
11th Dark Sky Symposium, Osnabruck Oct 6, 2011
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Verlust der Nacht(Loss of the Night)
• Collaboration of 9 Universities / Institutes
• 14 Integrated Subprojects
• Many aspects of light pollution considered, from measurement to ecology to sociology
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http://userpage.fu-berlin.de/~kyba/Light pollution conferences
Twitter: @skyglowberlinYoutube: skyglowberlin
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Outline
• Polarization of light
• Nocturnal insect navigation
• Experiment setup
• Results and interpretation
• Ecological consequences
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How does light become polarized?
• Polarization is a characteristic of light
• Most sources produce unpolarized light
• Horizontally polarized light generated by reflections
• Linearly polarized light generated through Rayleigh scattering
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How does light become polarized?
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Nocturnal Navigation
Dacke et al. 2003
• Dung beetles navigate using the polarized sky light of the moon
• Signal strength seven orders of magnitude smaller than in daytime
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Properties of sky polarization
• Degree of linear pol. is strongest at twilight• Twilight band of maximum polarization
runs approximately North/South• Moonlight polarization pattern almost
identical to sunlight p.p. (Gál et al. 2001)• Polarization pattern is visible in partly
cloudy skies (Pomozi et al. 2001)
=> Polarization is a more robust directional signal than the sun, moon, or stars
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Experimental Setup
• Berlin is at 52oN• North Star is 38o from zenith• Moon’s max deviation from
ecliptic is 18o-28o
• Moon is always 62o to 118o from North Star (66o to 114o in 2010)
• North Star is always near peak of Rayleigh polarization
• Geometry is independent of position on Earth, and changes a few degrees/day
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Measurement Locations
50 km
• Compare lunar skylight polarization at urban and rural location
• Brandenburg is very dark compared to Berlin
N
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Measurement equipmentSigma 24mm f1.8
tripod
SC4022LPFCFW-8
Blue Red
Note: Measure each LPF position (at least) 4x for each filter
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Skyglow pollutes lunar polarization signal!
Degree of linear polarization
Urban moonrise: 3.9 ± 0.2%Urban with moon: 11.3 ± 0.3%Rural moon: 29.2 ± 0.8%Urban daytime: 56.6 ± 1.0%Laboratory (LCD): 98.1 ± 1.2%
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Skyglow pollutes lunar polarization signal!
Degree of linear polarizationUrban no moon: 8.6 ± 0.3%Urban moonrise: 3.9 ± 0.2%Urban with moon: 11.3 ± 0.3%Rural moon: 29.2 ± 0.8%Urban daytime: 56.6 ± 1.0%Laboratory (LCD): 98.1 ± 1.2%
?!?!?
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Should skyglow be polarized?
• Most sources of light pollution are unpolarized
• Horizontally polarized light scattered upwards
• Rayleigh scattering can direct light downwards
• Sources of light pollution are spatially distributed
• Light pollution is generally uncollimated
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Naively, no
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What do simulations say?
• Skyglow simulations describe propagation of light from sources to observer
• Most do not take polarization into account
• The simulation that does (Kerola 2006) predicts that skyglow is almost unpolarized (~2%)
Aubé 2007
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But skyglow can be polarized!
• Moonless, clear sky observing conditions
• Similar values observed several months apart
• Results for one particular direction in one city
Degree of linear polarization370-510 nm: 10.1 ± 0.5%490-580 nm: 9.4 ± 0.7%590-690 nm: 8.5 ± 1.4%370-700 nm: 8.6 ± 0.3%
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How can this be?
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Ecological consequences
• Nocturnal insect navigation– dung beetles– crickets?– moths?– bees?
• Bird attraction to searchlights?
Dacke et al. 2003
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Conclusions
• Skyglow pollutes the natural polarization signal of the moon
• Extremely likely to affect navigational abilities of some nocturnal insects
• Skyglow itself can be weakly polarized
• Skyglow polarization could be used for remote sensing of aerosols at night
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Acknowledgements
Photo CreditsTable: Briho (Wikimedia Commons)
Sky: Christopher KybaDragonfly: Andreas Trepte (WC)Waggle dance: Jüppsche (WC)
Skyglow: Jeremy StanleyGlacier National Park: Ray Stinson
New York City: CharlieBrown7034 (WC)Light pollution map: WEW/FU Berlin
Light pollution model: Martin Aubé (2007)
FundingBMBF 033L038A
MILIEU (FU Berlin)
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Thank you!
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Polarization of daytime skylight
Rayleigh scattering
(Pomozi et al. 2001)
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Measuring Stokes Vector
When this is done for every pixel, you have imaging polarimetry
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SearchlightsVisual brightness
(blue band)Degree of
linear polarization
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Animal use of polarized light:Migration
• Birds use polarization cues to re-calibrate their magnetic compass daily
• Average of sunset and sunrise is true North, and independent of latitude and time of year
• Experiments hold birds in altered magnetic field during twilight
• Birds look for this cue preferably at the horizon
Mulheim et al. 2006Cochran et al. 2004
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Animal use of polarized light:Material detection
• Water detection is most well known use
• Leads to “polarized light pollution” from artificial surface reflections (Horváth et al. 2009)
• Acquatic animals also use it in hunting